The Nobel Committee said he won it for "his extraordinary efforts to strengthen international diplomacy and co-operation between peoples".
The committee highlighted Mr Obama's efforts to support international bodies and promote nuclear disarmament.
Mr Obama - woken up with the news early on Friday - said in an address at the White House that he was "surprised and deeply humbled" by the award.
He said he did not feel he deserved to be in the company of some of the "transformative figures" who had previously received the award.
Speaking outside the White House, he said he would accept the prize as a "call to action".
There were a record 205 nominations for this year's peace prize. Zimbabwean Prime Minister Morgan Tsvangirai and Chinese dissident Hu Jia had been among the favourites.
Instead the committee chose Mr Obama, who was inaugurated less than two weeks before the 1 February nomination deadline.
There was widespread surprise at the committee's decision. While world leaders were largely supportive of the award, thousands of people have contacted the BBC with more sceptical views.
An estimated 75% of comment sent to the BBC expressed surprise at the award. Some said awarding the prize to Mr Obama was plain wrong, others that the decision had come too soon, before he had made any concrete foreign policy achievement.
The Nobel laureate - chosen by a five-member committee - wins a gold medal, a diploma and 10m Swedish kronor ($1.4m).
"Only very rarely has a person to the same extent as Obama captured the world's attention and given its people hope for a better future," the Norwegian committee said in a statement.
"His diplomacy is founded in the concept that those who are to lead the world must do so on the basis of values and attitudes that are shared by the majority of the world's population."
Asked why the prize had been awarded to Mr Obama less than a year after he took office, Nobel Committee head Thorbjoern Jagland said: "It was because we would like to support what he is trying to achieve".
"It is a clear signal that we want to advocate the same as he has done," he said.
He specifically mentioned Mr Obama's work to strengthen international institutions and work towards a world free of nuclear arms.
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visit the BBC site with videos
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http://news.bbc.co.uk/2/hi/video_and_audio/news_summary/8070822.stm
http://news.bbc.co.uk/2/hi/europe/8298580.stm
US President Barack Obama has been awarded the 2009 Nobel Peace Prize.
The Nobel Committee said he won it for "his extraordinary efforts to strengthen international diplomacy and co-operation between peoples".
The committee highlighted Mr Obama's efforts to support international bodies and promote nuclear disarmament.
Mr Obama - woken up with the news early on Friday - said in an address at the White House that he was "surprised and deeply humbled" by the award.
He said he did not feel he deserved to be in the company of some of the "transformative figures" who had previously received the award.
Speaking outside the White House, he said he would accept the prize as a "call to action".
There were a record 205 nominations for this year's peace prize. Zimbabwean Prime Minister Morgan Tsvangirai and Chinese dissident Hu Jia had been among the favourites.
Instead the committee chose Mr Obama, who was inaugurated less than two weeks before the 1 February nomination deadline.
There was widespread surprise at the committee's decision. While world leaders were largely supportive of the award, thousands of people have contacted the BBC with more sceptical views.
An estimated 75% of comment sent to the BBC expressed surprise at the award. Some said awarding the prize to Mr Obama was plain wrong, others that the decision had come too soon, before he had made any concrete foreign policy achievement.
The Nobel laureate - chosen by a five-member committee - wins a gold medal, a diploma and 10m Swedish kronor ($1.4m).
"Only very rarely has a person to the same extent as Obama captured the world's attention and given its people hope for a better future," the Norwegian committee said in a statement.
"His diplomacy is founded in the concept that those who are to lead the world must do so on the basis of values and attitudes that are shared by the majority of the world's population."
Asked why the prize had been awarded to Mr Obama less than a year after he took office, Nobel Committee head Thorbjoern Jagland said: "It was because we would like to support what he is trying to achieve".
"It is a clear signal that we want to advocate the same as he has done," he said.
He specifically mentioned Mr Obama's work to strengthen international institutions and work towards a world free of nuclear arms.